The present invention relates to a new process for the decomposition of polyurethane plastics.
It is known that plastics produced by the polyisocyanate addition polymerization process can be broken down into low molecular weight constituents and converted into a soluble form by suitable reactive solvents. Particular attention has been given to the glycolytic decomposition of polyurethane plastic and foam waste materials, sometimes with the addition of amines, amino alcohols and/or catalysts. Such decomposition processes are disclosed, for example, in German Auslegeschriften or Offenlegungsschriften [patent applications laid open to public inspection] 1,110,405; 2,238,109; 2,304,444; 2,414,091; 2,516,863; 2,557,172; 2,738,572; 2,759,054; 2,902,509; 3,435,014 and 3,702,495; and U.S. Pat. Nos. 3,632,530; 4,014,809; 4,110,266; 4,159,972 and 4,162,995.
In view of the large number of disclosures directed to the decomposition of such materials, it is apparent that none of the known processes provides a satisfactory solution to the problem. In fact, the breadth of commercial application and the cost-effectiveness of the processes described in the prior art are restricted due to one or more of the following disadvantages:
1. The dissolution or reaction periods, which generally last for several hours, result in unsatisfactory space-time yields and limit the cost-effectiveness.
2. The amounts of diol required for the dissolution or decomposition of polyurethane plastic waste materials are often equal to or far in excess of the amount of waste material to be decomposed which creates a burden with respect to the amount of the decomposition fraction to be recycled.
3. The long reaction times (2 to 12 hours) and reaction temperatures of up to 250.degree. C. result in unwanted side reactions and in degradation of the regenerated polyols.
4. The use of catalysts in the decomposition reaction leads to difficulties when the regenerated polyols are subsequently used in an isocyanate addition polymerization process because these catalysts remain in the regenerated polyol and result in diverse catalytic effects (e.g., trimerization or dimerization of the isocyanates or conversion of the isocyanate into a carbodiimide, etc.).
5. Extraneous polymers present in the polyurethane waste material, particularly thermoplastics such as polypropylene, ABS, polystyrene or polycarbonate are not decomposed under glycolysis conditions. These extraneous polymers melt and contaminate or damage installations and lines.
6. During the glycolysis of polyurethane plastic waste materials of low density, such as hard and/or soft foams, in conventional stirred vessels, a large volume of foam (density about 40 to 80 g/l) must be dissolved in a small volume of glycol (density about 1100 g/l).
In order to circumvent the above-enumerated problems, various processes which avoid these disadvantages have been described.
DE 3,232,461 discloses a process for the continuous glycolytic decomposition of polyurethane plastic waste materials in screw-conveying machines. This process avoids the above-enumerated disadvantages and also makes it possible to process polyurethane plastic waste materials of low density. The disadvantages of this process are the high cost of creating and operating a screw reactor of this type and the peripheral devices associated therewith.
DE-C 4,030,639 and DE-A 4,132,282 describe the use of a multi-chamber gyro device (ultrasonic mill) for liquefying polyurethane plastic waste material steeped in solvents. These processes are limited to polyurethane foamed waste material because the material has to be steeped in suitable solvents before liquefaction. The material to be regenerated in these processes must therefore have a large surface area. In contrast, solid polyurethane plastic waste materials have to be ground to small particle sizes, which makes the cost-effectiveness of the process open to question.
Another disadvantage of the ultrasonic milling process is that polyurethane plastic waste materials are capable of absorbing large amounts of solvents such as polyols without forming a liquid mixture that can be pumped. This phenomenon is particularly pronounced with cellular polyurethane plastic waste materials (foamed materials) which, like a sponge, can absorb many times their own weight of solvent without forming a product that is capable of flow or that can be pumped. This means that only a small recycle fraction can be obtained.
One possible way of circumventing this problem is described in EP-A 546 415. In this disclosed process, the polyurethane plastic waste materials are glycolyzed with a large quantitative excess of polyol. This excess polyol is subsequently distilled off. This process consumes more energy than other prior art processes because (i) a relative excess of polyol has to be heated to the glycolysis temperature, and (ii) the excess polyol subsequently has to be removed by distillation.